Biaxially oriented polyester film with high oxygen barrier, its use, and process for its production

Abstract

A biaxially oriented polyester film is disclosed which has a base layer of at least 80% by weight of a thermoplastic polyester, and which has a cover layer and, located on the cover layer, a layer functioning as an O2 barrier, wherein said cover layer is composed of a polymer or of a mixture of polymers which comprises at least 40% by weight of ethylene 2,6-naphthalate units; 0% up to 40% by weight of ethylene terephthalate units; and, if desired, up to 60% by weight of units from other aliphatic, cycloaliphatic or aromatic diols and/or aliphatic, cycloaliphatic or aromatic dicarboxylic acids; with the proviso that the Tg2 value of the polyester film is above the Tg2 value of the base layer but below the Tg2 value of the cover layer. The film has particularly low atmospheric oxygen transmission and has high gloss. It is particularly suitable for packaging light- and air-sensitive foodstuffs and other light- and air-sensitive consumable items, and also for producing insulating materials.

What is claimed is:  
     
       1. A biaxially oriented polyester film having a base layer of at least 80% by weight based on the total weight of the base layer of at least one thermoplastic polyester, and having a cover layer and, located on the cover layer, a layer functioning as an O 2  barrier, wherein said cover layer is about 0.2 to about 4.5 μm thick and includes a polymer which comprises at least 40% by weight of ethylene 2,6-naphthalate units; and ethylene terepthalate units at greater than 0% up to 40% by weight; and if desired, 0% up to 60% by weight of units from other aliphatic, cycloaliphatic or aromatic diols and/or aliphatic, cycloaliphatic or aromatic dicarboxylic acids; with the proviso that the T g 2 value of the polyester film is above the T g 2 value of the base layer but below the T g 2 value of the cover layer; and wherein said base layer includes at least 90 mol percent of a component selected from the group consisting of polyethylene naphthalate, polyethylene terephthalate, and a combination thereof. 
     
     
       2. A film as claimed in  claim 1 , wherein the polymers of the cover layer comprise at least 65% by weight of ethylene 2,6-naphthalate units and up to 35% by weight of ethylene terephthalate units. 
     
     
       3. A film as claimed in  claim 1 , wherein the cover layer is pigmented. 
     
     
       4. A film as claimed in  claim 1 , wherein the film encompasses an additional cover layer on the side of the base layer facing away from the layer functioning as an O 2  barrier. 
     
     
       5. A film as claimed in  claim 4 , wherein the additional cover layer is pigmented. 
     
     
       6. A film as claimed in  claim 1 , wherein an intermediate layer is also located between the base layer and the cover layer. 
     
     
       7. A film as claimed in  claim 1 , having an overall thickness of from 4 to 100 μm, the base layer making up from 40 to 90% of the overall film thickness. 
     
     
       8. A film as claimed in  claim 1 , wherein the layer functioning as an O 2  barrier is a metallic layer and this metallic layer has an optical density in the range from 2.2 to 2.8. 
     
     
       9. A film as claimed in  claim 1 , wherein the layer functioning as an O 2  barrier is composed of a material selected from the group consisting of aluminum, silicon, SiO x , AlO x  and MgO x . 
     
     
       10. A film as claimed in  claim 1 , wherein at a pressure of 1 bar less than 0.3 cm 3  of atmospheric oxygen per square meter and day can diffuse through the film. 
     
     
       11. A film as claimed in  claim 1 , wherein the polymers of the cover layer comprise at least 70% by weight of ethylene 2,6-naphthalate units and up to 30% by weight of ethylene terephthalate units. 
     
     
       12. A film as claimed in  claim 1 , wherein the cover layer has a thickness of from 0.2 to 4.5 μm. 
     
     
       13. A film as claimed in  claim 1 , wherein the cover layer has a thickness of from 0.3 to 4 μm. 
     
     
       14. A film as claimed in  claim 1 , having an overall thickness of from 5 to 50 μm, the base layer making up from 40 to 90% of the overall film thickness. 
     
     
       15. A film as claimed in  claim 1 , having an overall thickness of from 6 to 30 μm, the base layer making up from 40 to 90% of the overall film thickness. 
     
     
       16. A film as claimed in  claim 1 , wherein at a pressure of 1 bar less than 0.25 cm 3  of atmospheric oxygen per square meter and day can diffuse through the film. 
     
     
       17. A film as claimed in  claim 1 , wherein at a pressure of 1 bar less than 0.2 cm 3  of atmospheric oxygen per square meter and day can diffuse through the film. 
     
     
       18. A biaxially oriented polyester film having a base layer of at least 80% by weight based on the total weight of the base layer of at least one thermoplastic polyester, and having a cover layer and, located on the cover layer, a layer functioning as an O 2  barrier, wherein said cover layer includes a polymer which comprises at least 40% by weight of ethylene 2,6-naphthalate units; and ethylene terephthalate units at greater than 0% up to 40% by weight; and if desired, 0% up to 60% by weight of units from other aliphatic, cycloaliphatic or aromatic diols and/or aliphatic, cycloaliphatic or aromatic dicarboxylic acids; with the proviso that the T g 2 value of the polyester film is above the T g 2 value of the base layer but below the T g 2 value of the cover layer; and wherein said base layer is about 40 to about 90% of a total thickness of said film and includes at least 90 mol percent of a component selected from the group consisting of polyethylene naphthalate, polyethylene terephthalate, and a combination thereof.